Method of electroslag remelting of consumable electrodes

ABSTRACT

A METHOD OF ELECTROSLAG REMELTING OF CONSUMABLE METAL ELECTRODES IN A COOLED MOLD, WHEREIN SHORT IMPULSES OF CURRENT ARE IMPRESSED UPON THE WORKING CURRENT OF AN ELECTRODE, THE AMPLITUDE AND DURATION OF SAID PULSES BEING SUFFICIENT TO EFFECT POSITIVE BREAKING OFF OF DROPS OF THE ELECTRODE METAL DURING THE FORMATION OF THE DROP PRIOR TO THE TIME AT WHICH IT WOULD NATURALLY DROP.

METHOD OF ELECTROSLAG REMELTING 0F CONSUMABLE ELECTRODES Original FiledJune 8, 1967 I I I I I I I I I I I I I I 1972 a. E. PATON E L 2Sheets-Sheet 1 I l I 4 y fi- J FIG] Aug. 8, 1972 a. a. PATON EI'AL3,682,622

METHOD OF ELECTROSLAG REMELTING 0F CONSUHABLE ELECTRODES Original FiledJune 8. 1967 2 Sheets-Sheet a 4 9/ 2 F163 )7 l l United States PatentUS. CI. 75-10 18 Claims ABSTRACT OF THE DISCLOSURE A method ofelectroslag remelting of consumable metal electrodes in a cooled mold,wherein short impulses of current are impressed upon the working currentof an electrode, the amplitude and duration of said pulses beingsufiicient to etfect positive breaking off of drops of the electrodemetal during the formation of the drop prior to the time at which itwould naturally drop.

This is a continuation of application Ser. No. 644,719, filed June 8,1967, now abandoned.

The present invention relates to methods of electroslag remelting ofconsumable electrodes and to installations for effecting said methods.

The invention is intended to be used in electrometallurgy, in particularwhen making high-quality steels and alloys by the method of electroslagremelting.

Known in the prior art are methods of electroslag remelting ofelectrodes in a cooled mould with the use of a direct and alternatingcurrent, and installations for eflecting said methods, in which themetal of the electrode, when passing dropwise through the layer ofmolten slag, is cleaned from non-metallic inclusions, the latter beingabsorbed by the slag. Besides, the metal drops, when forming anddropping, are subjected to a partial cleaning from sulphur, phosphorusand gases (see the book Electroslag Remelting, by B. I. Medovar, Yu. V.Iatash, Kiev, 1963, pp. 48 and 49.

It has been established that smaller drops of metal when passing throughthe layer of molten slag are likely to be cleaned quicker and morecompletely from non-metallic inclusions and other impurities than it ispossible to achieve with larger drops.

In the existing methods of electroslag remelting and in theinstallations .for effecting same, the size of drops is determined bynatural conditions of the melting process, the size of drops beingsufiiciently large, with the diameter ranging from to 12 mm. For thisreason, the exchange reactions occurring between the metal of the dropand the slag are insufficiently vigorous, and the possibilities of theprocess of electroslag remelting are thus not completely used toadvantage.

An object of the present invention is to eliminate the above-saiddisadvantage.

The principal object of the present invention is to develop such amethod of electroslag remelting of consumable electrodes and to providesuch an installation for efiecting same, which would provide for a morecomplete cleaning of the metal being remelted from undesirableimpurities owing to a considerable decrease in the size of drops of theelectrode metal by way of positive breaking off of the drops prior tothe moment of their being naturally broken ofi.

This object is accomplished by superimposing short pulses of current onthe working current of the electrode,

the amplitude and duration of said pulses being fairly sufficient forthe positive breaking off of the drops of electrode metal during theformation of the drop up to the moment of its being naturally brokenoff, which is taking place in the process of the electroslag remeltingof consumable electrodes in a cooled mould.

In conformity with the present invention, the installation for eliectingthe method proposed herein is provided with a pickup determining themoment of formation of the electrode metal drops, the input of saidpickup being connected to a current measuring instrument, while itsoutput is connected to a time relay actuating a control unit of thedevice setting up current pulses in the circuit of the consumableelectrode.

In one of the embodiments of the installation for effecting theherein-proposed method, an ignitron contactor may be employed as thedevice for setting up current pulses in the circuit of the consumableelectrode, said ignitron contactor being connected in series with asuitable source of current adapted to feed the circuit of the eonsumableelectrode.

The nature of the present invention will become more fully apparent froma consideration of the following description of a preferred embodimentof the method of electroslag remelting of consumable electrodes, takenin conjunction with the accompanying drawings illustrating one possibleembodiment of the installation for efiecting the proposed method, inwhich:

FIG. 1 represents a consumable electrode at the moment of a molten metaldrop naturally breaking off;

FIG. 2 shows the electrode at the moment when the drop of molten metalis positively broken off;

FIG. 3 represents an oscillogram of alternating current of the furnacewith the natural breaking olf of metal drops from the end of theconsumable electrode;

FIG. 4 shows the oscillogram with the positive breaking oil of metaldrops from the end of the consumable electrode;

FIG. 5 is a schematic diagram of the installation for the electroslagremelting effected according to the method of the invention; and

FIG. 6 represents an oscillogram of current, passing through theignitron contactor of the proposed installation. The essence of theproposed method is as follows. In the process of the electroslagremelting of consumable electrodes, the metal to be remelted is cleanedfrom undesirable impurities by making it interact with the molten slag.Among other factors, the degree of cleaning of the metal being remeltedis determined by the magnitude of the surface of contact between theliquid metal and slag. A decrease in the size of drops signifies anincrease in this surface, and is likely to speed up the processes ofinteraction of metal of the drop with the slag.

The liquid metal is retained at the electrode end by the force ofsurface tension F (FIG. 1) and hydrostatic lifting force F The force ofgravity P and electrodynamic force F tend to separate the metal to bemelted in the form of drops from the electrode end (FIG. 1). In theprocess of the electroslag remelting up to the moment of the naturalbreaking off of the metal drop, there is observed an inequality.

At the moment when the drop is naturally broken off, the forces tendingto separate the drop from the end of the electrode (P-l-F exceed theforces retaining the drop at the end of the electrode Under the naturalconditions of the melting process, this inequality is observed when thesize of the drop is rather considerable (up to 10 to 12 mm. indiameter), mainly on account of an increase in the weight, and hence inthe size of the drop.

The component of the electrodynamic force F compressing the electrodeand tending to separate the molten metal therefrom, is directeddownwards and proportional to the square of the current. When remeltingwith the use of an alternating current, F varies with time according tothe following law:

where c is the coefficient of proportionality,

'r is the amplitude of alternating current,

w is the angular frequency of the current and t is the current moment oftime.

From Equation 3 for the force F it is evident that when remelting withthe use of an alternating current of the angular frequency w, the forceF is likely to preserve constant the direction of action, sin-cc cos 2wt=l, and varies from zero to F =C-r In order to decrease the size ofdrops of the electrode metal, i.e. to break off positively drops ofsmall size, it is necessary, as it becomes evident from Equation 2, toincrease for a short time the electrodynamic force F up to the value F(FIG. 2) which must be sufficient for effecting positive breaking ofl ofthe drop of electrode metal during the formation of the drop up to themoment of its natural breaking 05.

An increase in the force F is achieved due to the superimposing of shortpulses of current upon the working current of the furnace, the amplitudeand duration of said current pulses being suflicient to effect thepositive breaking off of small drops which is illustrated in FIGS. 3 and4.

FIG. 3 shows an oscillogram of the alternating working current with thenatural breaking off of drops of molten metal from the end of aconsumable electrode. The current of frequency A equal to 50 cycles persecond, when passing through the electrode and melting down the slag, ismodulated by the low frequency B. This is caused by the fact that thedrop growing at the end of the electrode shunts the interelectrode gap,whose resistance determines the current of the furnace. Hence, on theoscillogram (FIG. 3) the minimum value of the amplitude of the workingcurrent T1 in each period T corresponds to the absence of the drop atthe end of the electrode, while the maximum value of the amplitude ofthe working current 1-; corresponds to the moment immediately precedingthe natural breaking off of the drop.

Thus, at each moment of time of each period the increment of theamplitude of the working current A-r is determined from the equation:

Where 1- is the value of the working current amplitude at a certainmoment of time of the period under consideration;

r; is the minimum value of current amplitude in the period underconsideration, characterizing the size of the metal drop growing at theelectrode end.

When Ar=A =(1- n), there occurs natural breaking off of the drop fromthe end of the consumable electrode.

When remelting with the use of a direct current, the working current ofthe electrode is also likely to vary periodically as a result ofshunting of the interelectrode gap by the drops of molten metal, andthese variations of the working current during each period alsocharacterize the size of the metal drop growing at the electrode end.

To effect positive breaking off of small drops of the electrode metal,pulses C of current (FIG. 4) are to be superimposed upon the workingcurrent, the current having the amplitude 'r 1' at the moments when(FIG. 4). An abrupt increase in the electrodynamic force F acting uponthe molten metal at the electrode end, results in the positive breakingoff of drops of molten metal. The magnitude of the pulse amplitude andduration T are to be selected for each particular case, on account ofthe complicated calculations required, so that said values be sufficientto effect positive breaking off of metal drops.

When comparing oscillograms represented in FIGS. 3 and 4, it becomesevident that the frequency of breaking off of metal drops when operatingaccording to the proposed method is likely to increase, though the speedof the melting process remains constant, on account of which the surfaceof contact between the molten metal and liquid slag increases, whichcontributes to a more complete elimination of undesirable impuritiesfrom the metal being remelted.

One of the possible embodiments of the installation for effecting theproposed method of electroslag remelting of consumable electrodes iscomprised of a furance 2 (FIG. 5) for electroslag remelting with the useof an electrode 1 immersed into molten slag 3, a power transformer 4, anignitron contactor 5, a unit 6 for controlling and igniting the ignitronof the ignitron contactor, a pickup 7 indicating the moment of formationof drops of the electrode metal, a current transformer 8, and acontactless time relay 9.

The installation represented in the drawing operates in the followingmanner: The ignitron contactor 5 is switched on and is operated by theunit 6 for controlling and igniting the ignitrons of the ignitroncontactor in a continuous mode the ignition angle a. (FIG. 6) equalling,for example, 70 to and ensuring the working current of the furnacehaving the amplitude D. A signal from the current transformer 8 issupplied to the pickup 7 indicating the moment of formation of theelectrode metal drops.

At the moment 0 of beginning of the formation of the molten metal dropat the end of the electrode 1, as determined by the pickup 7 accordingto the value of the increment of the working current amplitude A=(r1-(see Equation 4), a pulse C is produced at the output of the pickup 7,which is fed to the contactless time relay 9, synchronized with thenetwork. The time relay 9 produces a rectangular pulse of a durationequal to one or several periods of the supply network current, saidpulse being fed to the unit 6 adapted for controlling and igniting theignitrons of the ignitron contactor 5.

During the supply of the pulse C there occurs an abrupt variation of theignition angle a igniting the ignitrons from the working (a=70 to 90) tothe full-phase current (a=0), on account of which an abrupt increase inthe current up to the intensity 1- with the amplitude E takes place inthe secondary circuit of the power transformer 4 supplying the furnace.The duration of the current pulse C is equal to the duration of thepulse fed to the control unit 6 from the time relay 9.

This pulse causes an increase in the electrodynamic force F which breaksoff from the end of the electrode 1 a portion of molten metal in theform of a small drop.

There are also possible other embodiments of the installations foreffecting the proposed method of electroslag remelting of consumableelectrodes with the use of either alternating or direct current.

When carrying out the electroslag remelting according to the proposedmethod on the installation of the invention, there is achieved aconsiderable increase in the efficiency of treating metal with slag,owing to which the remelted metal possesses a lower percentage ofsulphur, phosphorus, gases, non-metallic inclusions and other harmfulimpurities than it is possible to obtain by means of other known methodsof electroslag remelting on the existing installations.

What is claimed and desired to be secured by Letters Patent is:

1. A method of electroslag remelting of consumable metal electrodes,said method comprising immersing a consumable electrode into moltenslag, supplying a working current to said electrode to cause theformation of drops of the material of the electrode at the lower endthereof, and intermittently impressing on said working current impulsesof increased amplitude and of short duration relative to the intervalsbetween said impulses, said impulses being of an amplitude and of aduration to produce removal of the drops at a time earlier than that atwhich they would naturally fall from the electrode and of a size smallerthan that at which they would naturally fall from the electrode.

2. A method of electroslag remelting comprising forming a bath of moltenslag, immersing a consumable electrode in said bath of molten slag,passing electric working current between said electrode and said bath ofmolten slag to cause the immersed end of said electrode to melt and toform drops of liquid metal on said electrode, impressing on said workingcurrent impulses of increased amplitude and of short duration relativeto the intervals between said impulses, the amplitude and duration ofsaid impulses being sulficient to effect positive breaking off of saiddrops of liquid metal from said electrode during formation of each dropprior to the time such drop would naturally fall from said electrode.

3. A method of electroslag remelting as recited in claim 2, wherein saidworking current is alternating current.

4. A method of electroslag remelting as recited in claim 3, wherein saidimpulses are alternating current impulses.

5. A method of electroslag remelting as recited in claim 4, furthercomprising connecting in the path of said working current a circuitelement which is controllable to permit conduction only during apredetermined portion of each cycle of said working current and whereinsaid impulses are impressed on said working current by increasing thepredetermined portion of each cycle that conduction is permitted by saidcircuit element.

6. A method of electroslag remelting as recited in claim 2, furthercomprising detecting when the working current increases to apredetermined value, and impressing said impulses on said workingcurrent upon detection that said working current has increased to saidpredetermined value.

7. A method of electroslag remelting comprising: forming a bath ofmolten slag; immersing at least an end portion of a consumable electrodein the bath of molten slag; supplying workable electrical power to theelectrode so that it passes from the immersed portion of the electrodethrough molten slag to cause the end portion of the electrode to meltand to form drops of liquid material from the electrode on the immersedend portions; impressing on said working electrical power, increments ofelectrical power additive to that of said working electrical power, theelectrical power of said increments being sufiicient to elfect positiveseparation of said drops of liquid material from the immersed part ofthe electrode prior to the time which is normally required for a liquidmaterial drop to form and to normally fall from the electrode as aresult of normal working electrical power.

8. A method of electroslag remelting as set forth in claim 7, whereinthe working electrical power is alternating current.

9. A method of electroslag remelting as set forth in claim 8, whereinthe increments of added electrical power are alternating currentimpulses.

10. A method of electroslag remelting as set forth in claim 9, furthercomprising: connecting in the path of the working electrical power acircuit element which is controllable to permit working alternatingcurrent conduction only during a predetermined conduction angle of theworking current and wherein the impulses are impressed on the workingcurrent by increasing the conduction angle permitted by the circuitelement.

11. A method of electroslag remelting as set forth in claim 7, furthercomprising detecting when the working electrical power increases to apredetermined value, and impressing the increments upon detection thatthe working electrical power has increased to the predetermined value.

12. A method of electroslag remelting as set forth in claim 8, furthercomprising making the impulses of additive electrical power of increasedamplitude over that of the working electrical power and of shortduration relative to the time period of the impulses.

13. A method of electroslag remelting comprising: forming a bath ofmolten slag; immersing at least an end portion of a consumable electrodein the bath of molten slag; supplying workable electrical power to theelectrode so that it passes from the immersed portion of the electrodethrough molten slag to cause the end portion of the electrode to meltand to form drops of liquid material from the electrode on the immersedend portion; impressing on said working electrical power, increments ofelectrical power additive to that of said working electrical power, theelectrical power of said increments being sufficient to effect positiveseparation of said drops of liquid material of less than 10 mm. diameterfrom the immersed part of the electrode.

14. A method of electroslag remelting as set forth in claim 13, whereinthe working electrical power is alternating current.

15. A method of electroslag remelting as set forth in claim 14, whereinthe increments of added electrical power are alternating currentimpulses.

16. A method of electroslag remelting as set forth in claim 15, furthercomprising: connecting in the path of the working electrical power acircuit element which is controllable to permit working alternatingcurrent conduction only during a predetermined conduction angle of theworking current and wherein the impulses are impressed on the workingcurrent by increasing the conduction angle permitted by the circuitelement.

17. A method of electroslag remelting as set forth in claim 13, furthercomprising detecting when the working electrical power increases to apredetermined value, and impressing the increments upon detection thatthe working electrical power has increased to the predetermined value.

18. A method of electroslag remelting as set forth in claim 14, furthercomprising making the impulses of additive electrical power of increasedamplitude over that of the working electrical power and of shortduration relative to the time period of the impulses.

References Cited UNITED STATES PATENTS 1,967,872 7/1934 Doan -123 R X2,504,868 4/1950 Muller 219l37 X 3,047,708 7/1962 Stark 219-146 X L.DEWAYNE RUTLEDGE, Primary Examiner I E. LEGRU, Assistant Examiner U.S.Cl. X.R. 219-73

